Real iridium.
Iridium metal is hard to get, and very expensive. In fact, it was in the very last group of elements I was able to acquire to complete my collection. It was donated by the extremely kind Max Whitby of the The Red Green & Blue Company, which sells a complete collection of elements.

Osmium and Iridium are the two densest elements in the world (they are in fact so close in density that which one is considered the densest has switched a couple of times over the years). Even though this is a quite small lump, you can feel its weight when you shake the bottle: Quite surprising. Having a large block of this would be remarkable, but the closest I'm likely to come to that is my large blocks and cylinders of tungsten, which are only about 15% less dense.